This invention relates to data over cable transmission and, more particularly, to a method and apparatus for generating requests to transmit data packets upstream from a cable modem to a cable headend.
To coordinate upstream data transmission from a number of cable modems to a headend in a cable transmission system so there is no collision among a number of cable modems using the same channel, the cable modems send request messages to the headend that tell the headend the amount of bandwidth needed to transmit the data. The bandwidth expressed in minislots is sometimes referred to as the physical length (PHY L) of the data to be transmitted. The PHY L required to transmit a data packet having a given byte length (BL) varies depending upon the overhead imposed by the physical layer of the cable system, such as the preamble, guard band, forward error correction, and padding. The parameters that determine the overhead are part of a burst profile transmitted to each cable modem through uniform channel descriptor (UCD) messages. When a request to transmit data is made by a modem, the BL of the data packet to be transmitted is converted to the PHY L by calculations based on a formula including the parameters. When a grant message is returned to the requesting modem, it tells the requesting modem which minislots to use for upstream transmission of the data packet.
If the cable transmission system is capable of fragmented data transmission, i.e., transmitting a data packet in two or more fragments, the grant message sent from the headend to the requesting cable modem includes the PHY L of the data segment to be transmitted in the allotted bandwidth. Before transmitting this data fragment, its PHY L must be converted to a BL value. This conversion is also made by calculations based on the formula including the parameters.